1. Field of the Invention
The present invention relates to a mobile communication system, such as automobile telephone system, portable telephone system, cordless telephone system, and radio LAN system.
2. Description of the Related Art
Mobile communication systems including automobile telephones and portable telephones are widely employed, and recently to cope with the increasing number of subscribers of such mobile communication systems, it has been attempted to dispose plural base stations in a service area, and use a same frequency between base stations at the shortest distance capable of repeatedly using the same frequency. That is, in the mobile communication systems such as automobile telephones and portable telephones, the location registration area is set in a wide area unit covering an entire state or a region of a state. In this area, plural base stations are disposed discretely, and which location registration area a specific mobile station is present can be registered at the system side. When a certain mobile station is called, the called mobile station is called at once from all base stations in that location registration area, and the communication channel between the caller and the mobile terminal is maintained through the base station receiving the reply from the called mobile station, so that the communication is established. When the caller is a mobile terminal, call origination is noticed to the system side through the base station receiving the call origination from this mobile station. At the system side, from the location registration information of the mobile station at the called mobile station, the called station is called at once from all base stations in the area where the terminal location is registered, and the communication channel between the caller and the called terminal is maintained through the base station receiving the reply from the called mobile station, thereby establishing the communication. Thus, communication of the mobile station is enabled.
In such mobile communication system, by reducing the size of cell covered by one base station, the radio frequency band can be used more efficiently, so that more subscribers can be enrolled.
FIG. 1 is a block diagram showing a constitution of a conventional mobile communication system. In the diagram, MS.sub.1 to MS.sub.m are mobile stations, C.sub.1 to C.sub.n are cells, BS.sub.1 to BS.sub.n are base stations, LRA.sub.1 to LRA.sub.x are location registration areas, 1 is a mobile communication control apparatus, and 2 is a mobile communication wired network. More specifically, mobile stations MS.sub.1 to MS.sub.m are movable terminals which can receive presentation of communication services by this mobile communication system.
Base stations BS.sub.1 to BS.sub.n are stations for radio communication discretely disposed in the coverage area of service of the mobile communication system, and cells C.sub.1 to C.sub.n are zones accessible by radio communication from any corresponding base station of the base stations BS.sub.1 to BS.sub.n. Herein, the region capable of receiving the radio wave from the base station BS.sub.1 is the cell C.sub.1, the region capable of receiving the radio wave from the base station BS.sub.2 is the cell C.sub.2, the region capable of receiving the radio wave from the base station BS.sub.3 is the cell C.sub.3, and so forth as shown correspondingly in the diagram.
The location registration areas LRA.sub.1 to LRA.sub.x are areas for management and control for calling a corresponding mobile station when the mobile station is called by registering the locations of the mobile stations in plural cells.
The mobile communication control apparatus 1 registers locations of mobile stations in every one of the location registration areas LRA.sub.1 to LRA.sub.x, pages a mobile station when the mobile station is called, changes over the communication route so that the communication of the mobile station may be continued through the base station to cover the cell at the moving destination according to the move of the mobile station across cells after the call is set up, and provides other controls. The mobile communication control apparatus 1 also includes a paging controller PMB responsible for paging control of the mobile station when a mobile station is called.
The operation of such conventional mobile communication system is described below.
Each base station periodically informs the mobile stations of location registration data. When the mobile station receives this information and detects that it is within a new location registration area, the mobile station transmits its own ID (identification code) corresponding thereto. When the base station receives the ID, it transmits it to the control apparatus 1. At the control apparatus 1, on the basis of the ID, the location registration area to which the received base station belongs is registered in every mobile station to be ready for call to the mobile stations.
Or, when the power source of each mobile station is turned on, when the mobile station is moved to another cell, or when the mobile station is come into the service area from the outside of the service area of the radio communication, each mobile station transmits own ID (identification code) together with the location registration request. The base station receives it and sends the information to the control apparatus 1. At the control apparatus 1, on the basis of the received information, the location registration area to which the received base station belongs is registered in each mobile station, to be ready for call to the mobile stations.
Thus, for call connection, the location of the mobile stations is registered, and when a mobile station is called, the control apparatus 1 controls to page within all cells in the location registration area in which the called mobile terminal is registered by the paging controller PMB.
As a result, the objective mobile station is called from all base stations in the location registration area in which the called mobile station is registered. When the called mobile station responds, it is received by a base station, which returns to the response to the control apparatus 1, and the control apparatus 1 keeps the communication route between the called mobile station and the caller's terminal through the receiving base station, so that the communication between the two parties is enabled.
In such a conventional mobile communication system, when a certain mobile station is called, a paging signal is transmitted at once from all base stations in all cells belonging to the location registration area in which the called mobile station is registered.
However, the called mobile station is located only in a specific cell in the location registration area, and can communicate only with the base station responsible for communication service of that cell, but cannot communicate with base stations in other cells. What's more, the location registration area is vast, and there are many base stations in the location registration area, and it is an utter waste to send paging signals by base stations in the cells in which the called mobile terminal is not present.
Every time the mobile station is called, such paging signal is transmitted from all base stations in the cells belonging to the location registration area in which the specific mobile station is registered, which is a mere increase traffic by useless call in other base stations.
To save such traffic in each base station and utilize the radio channel efficiently, it seems enough to narrow the location registration area, but when the location registration area is narrowed, in turn, it is highly possible that the mobile station soon moves to other location registration area, and the location must be registered frequently, and thus although the paging traffic is decreased, the location registration traffic increases.
Thus, relating to the size of the location registration area, a trade-off occurs between the location registration traffic and paging traffic. It has been hence considered to set the location registration area on the basis of the mobility of each mobile station and the incoming call rate.
Anyway, in such location registration method and paging method, the future multimedia possibility of mobile communication is not taken into consideration, and wasteful control traffic may occur when communications by various media have come to be used in the mobile communication system.
It is therefore demanded to develop a technology capable of reducing such wasteful control traffic at the time of call connection control.
Moreover, as described above, in the mobile communication systems such as automobile telephones and portable telephones employing the system of disposing plural base stations in a service area, and using the same frequency between base stations at the shortest distance capable of repeatedly using the frequency, to cope with the increasing number of subscribers, it has been attempted to utilize the radio frequency band more efficiently, by reducing the size of the cells formed by one base station, and shortening the distance allowing to use the same frequency repeatedly. As a result, more subscribers can be enrolled.
However, when the size of cell is smaller, it causes to increase the number of times of hand-off for changing over the cells during communication as the mobile station moves during communication. At the time of hand-off, the communication is interrupted for a moment, and as the coverage area of the cell becomes narrower, it causes to increase the control for hand-off.
To solve such problem, there is proposed a mobile communication system of hierarchical cell organization by disposing cells of different sizes in the service area. Such a mobile communication system is described below by referring to a block diagram in FIG. 2.
In the service area, plural first cells CS.sub.1 to CS.sub.n covering a geographically narrow range, and plural second cells CL.sub.1 to CL.sub.m covering a wider range geographically than the first cells are disposed. The first cells CS.sub.1 to CS.sub.n are composed of first base stations BSS.sub.1 to BSS.sub.n, and the second cells CL.sub.1 to CL.sub.m are composed of second base stations BSL.sub.1 to BSL.sub.m. A communication control apparatus 3 in the mobile communication system includes a movement detector 4 for controlling the mobile communication system depending on the moving speed of the mobile station. The mobile communication system presents communication service by the second cells of wider range to the mobile station moving at high speed, and presents communication service by the first cells of narrower range to the mobile station moving at low speed. In such mobile communication system, as compared with the mobile communication system covering the entire service area by plural cells of same coverage range, the number of times of hand-off can be decreased without sacrificing the utility efficiency of the radio frequency band.
However, as the mobile communication system for selecting the cells for presenting service to mobile stations, only the above mobile communication system operating on the basis of the moving speed of mobile station has been proposed so far, and nothing has been proposed about the mobile communication system capable of selecting the cells for presenting service to the mobile station on the basis of other parameters than the moving speed.
On the other hand, as the modern communication is in the trend of multimedia and personal use, it gives rise to need of transmitting in a system suited to individual types of information, in consideration of the types of information to be transmitted, particularly in the case of transmission of multimedia information. In the conventional mobile communication systems mentioned so far, however, no consideration is given to transmission of multimedia information, and it seems to cause various problems, including worsening of utility efficiency of radio channels, increase of control quantity of the radio network, and lowering of service quality.